While-drilling methodology for estimating formation pressure based upon streaming potential measurements

ABSTRACT

Logging-while-drilling apparatus and methodologies for measuring streaming potential in an earth formation are provided. The apparatus and methodologies can be utilized to find information relevant to the drilling operations. In particular, since the streaming potential measurement relates directly to fluid flow, the streaming potential measurements can be used to track flow of fluids in the formation. In turn, this information may be used to find information relevant to the drilling operations, such as under-balanced drilling conditions, abnormal formation pressures, open fractures, the permeability of the formation, and formation pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefits of priority from the following U.S.patent applications:

i) application Ser. No. 10/871,856, entitled “WHILE-DRILLING APPARATUSFOR MEASURING STREAMING POTENTIALS AND DETERMINING EARTH FORMATIONCHARACTERISTICS,” filed on Jun. 18, 2004;

ii) application Ser. No. 10/871,854, entitled “WIRELINE APPARATUS FORMEASURING STREAMING POTENTIALS AND DETERMINING EARTH FORMATIONCHARACTERISTICS,” filed on Jun. 18, 2004;

iii) application Ser. No. 10/871,446, entitled “COMPLETION APPARATUS FORMEASURING STREAMING POTENTIALS AND DETERMINING EARTH FORMATIONCHARACTERISTICS,” filed on Jun. 18, 2004; and

iv) application Ser. No. 10/872,112, entitled “METHODS FOR LOCATINGFORMATION FRACTURES AND MONITORING WELL COMPLETION USING STREAMINGPOTENTIAL TRANSIENTS INFORMATION,” filed on Jun. 18, 2004;

all of which are commonly assigned to assignee of the present inventionand hereby incorporated by reference in their entirety.

This patent application is also related to the followingcommonly-assigned U.S. patent applications which are hereby incorporatedby reference in their entirety:

-   -   i) No. ______, entitled “WHILE-DRILLING APPARATUS FOR MEASURING        STREAMING POTENTIALS AND DETERMINING EARTH FORMATION        CHARACTERISTICS AND OTHER USEFUL INFORMATION” (Attorney Docket        No. 60.1610);    -   ii) No. ______, entitled “WHILE-DRILLING APPARATUS FOR MEASURING        STREAMING POTENTIALS AND DETERMINING EARTH FORMATION        CHARACTERISTICS AND OTHER USEFUL INFORMATION” (Attorney Docket        No. 60.1633);    -   iii) No. ______, entitled “WHILE-DRILLING METHODOLOGY FOR        DETERMINING EARTH FORMATION CHARACTERISTICS AND OTHER USEFUL        INFORMATION BASED UPON STREAMING POTENTIAL MEASUREMENTS”        (Attorney Docket No. 60.1634);        each such U.S. patent application being simultaneously filed        herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to the hydrocarbon industry. Moreparticularly, this invention relates to apparatus and methods formeasuring streaming potentials resulting from pressure transients in anearth formation traversed by a borehole. This invention also relates tomanners of making determinations regarding earth formationcharacteristics as a result of streaming potential measurements. Onesuch characteristic is the permeability of the formation at differentdepths thereof, although the invention is not limited thereto.

2. State of the Art

Streaming potential, also commonly referred to as electrokineticpotential, is an electric potential generated by an electrolyte (e.g.,water) flowing through a porous medium. The history with respect to thepossibility of making streaming potential measurements in a borehole isa long one. In U.S. Pat. No. 2,433,746, Doll suggested that vigorousvibration of a downhole apparatus in a borehole could generate pressureoscillations and fluid movement relative to the formation which in turncould give rise to measurable streaming potentials due to anelectrokinetic potential phenomenon. In U.S. Pat. No. 2,814,017, Dollsuggested methods for investigating the permeabilities of earthformations by observing the differences in phase between periodicpressure waves passed through the formations and potentials generated bythe oscillatory motion of the formation fluid caused by these pressurewaves. Conversely, a periodically varying electric current was suggestedto be used to generate oscillatory motion of the formation fluid, whichin turn generated periodic pressure waves in the formation. Measurementswere to be made of the phase displacement between the generating and thegenerated quantities and a direct indication of the relativepermeability of the formation thereby obtained.

In U.S. Pat. No. 3,599,085, to A. Semmelink, entitled, “Apparatus ForWell Logging By Measuring And Comparing Potentials Caused By SonicExcitation”, the application of low-frequency sonic energy to aformation surface was proposed so as to create large electrokineticpulses in the immediate area of the sonic generator. In accordance withthe disclosure of that patent, the electrokinetic pulses result from thesqueezing (i.e. the competition of viscosity and inertia) of theformation, and the streaming potential pulses generate periodicmovements of the formation fluid relative to the formation rock. Thefluid movement produces detectable electrokinetic potentials of the samefrequency as the applied sonic energy and having magnitudes at any givenlocation directly proportional to the velocity of the fluid motion atthat location and inversely proportional to the square of the distancefrom the locus of the streaming potential pulse. Since the fluidvelocity was found to fall off from its initial value with increasinglength of travel through the formation at a rate dependent in part uponthe permeability of the formation rock, it was suggested that themagnitude of the electrokinetic potential at any given distance from thepulse provided a relative indication of formation permeability. Byproviding a ratio of the electrokinetic potential magnitudes(sinuisoidal amplitudes) at spaced locations from the sonic generator,from which electrokinetic skin depth may be derived, actual permeabilitycan in turn be determined.

In U.S. Pat. No. 4,427,944, Chandler suggested a stationary-typeborehole tool and method for determining formation permeability. Theborehole tool includes a pad device which is forced into engagement withthe surface of the formation at a desired location, and which includesmeans for injecting fluid into the formation and electrodes formeasuring electrokinetic streaming potential transients and responsetimes resulting from the injection of the fluid. The fluid injection iseffectively a pressure pulse excitation of the formation which causes atransient flow to occur in the formation. Chandler suggests ameasurement of the characteristic response time of the transientstreaming potentials generated in the formation by such flow in order toderive accurate information relating to formation permeability.

In U.S. Pat. No. 5,503,001 (1996), Wong proposed a process and apparatusfor measuring at finite frequency the streaming potential andelectro-osmotic induced voltage due to applied finite frequency pressureoscillations and alternating current. The suggested apparatus includesan electromechanical transducer which generates differential pressureoscillations between two points at a finite frequency and a plurality ofelectrodes which detect the pressure differential and streamingpotential signal between the same two points near the source of thepressure application and at the same frequency using a lock-in amplifieror a digital frequency response analyzer. According to Wong, because theapparatus of the invention measures the differential pressure in theporous media between two points at finite frequencies close to thesource of applied pressure (or current), it greatly reduces the effectof background caused by the hydrostatic pressure due to the depth of theformation being measured.

Despite the long history and multiple teachings of the prior art, it isbelieved that in fact, prior to field measurements made in support ofinstant invention, no downhole measurements of streaming potentialtransients in actual oil fields have ever been made. The reasons for thelack of actual implementation of the proposed prior art embodiments areseveral. According to Wong, neither the streaming potential nor theelectro-osmotic measurement alone is a reliable indication of formationpermeability, especially in formations of low permeability. Wong statesthat attempts to measure the streaming potential signal with electrodesat distances greater than one wavelength from each other are flawedsince pressure oscillation propagates as a sound wave and the pressuredifference would depend on both the magnitude and the phase of the wave,and the streaming potential signal would be very much lower sinceconsiderable energy is lost to viscous dissipation over such a distance.In addition, Wong states that application of a dc flow to a formationand measurement of the response voltage in the time domain will not workin low permeability formations since the longer response time and verylow streaming potential signal is dominated by drifts of the electrodes'interfacial voltage over time. Thus, despite the theoreticalpossibilities posed by the prior art, the conventional wisdom of thoseskilled in the art (of which Wong's comments are indicative) is thatuseful streaming potential measurements are not available due to lowsignal levels, high noise levels, poor spatial resolution, and poorlong-term stability.

Indeed, it is difficult to obtain pressure transient data with highspatial resolution as the borehole is essentially an isobaric region.The pressure sensor placed inside the borehole cannot give detailedinformation on the pressure transients inside the formation if theformation is heterogeneous. To do so, it is necessary to segment theborehole into hydraulically isolated zones, a difficult and expensivetask to perform. Further, it will be appreciated that some of theproposed tools of the prior art, even if they were to function asproposed, are extremely limited in application. For example, theChandler device will work only in drilled boreholes prior to casing andrequires that the tool be stationed for a period of time at eachlocation where measurements are to be made. Thus, the Chandler devicecannot be used as an MWD/LWD (measurement or logging while drilling)device, is not applicable to finished wells for making measurementsduring production, and cannot even be used on a moving string of loggingdevices.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide methods andapparatus for measuring streaming potential in an earth formation.

It is another object of the invention to provide methods and apparatusfor measuring streaming potentials in a formation while drilling aborehole.

It is an additional object of the invention to provide methods ofdetermining formation characteristics using streaming potentialsmeasurements.

Another object of the invention is to provide methods of identifyingdangerous drilling conditions, such as under-balanced drillingconditions and/or fluid loss from fractures, using streaming potentialmeasurements.

A further object of the invention is to provide methods of determiningformation permeability and/or formation pressure using streamingpotential measurements.

In accord with these objects, which will be discussed in detail below, alogging-while-drilling apparatus and methodologies for measuringstreaming potential in an earth formation are provided. For purposesherein, logging-while-drilling (LWD) applications andmeasurement-while-drilling (MWD) applications will be consideredinterchangeable. The apparatus and methodologies can be utilized to findinformation relevant to the drilling operations. In particular, sincethe streaming potential measurement relates directly to fluid flow, thestreaming potential measurements can be used to track flow of fluids inthe formation. In turn, this information may be used to find informationrelevant to the drilling operations, such as under-balanced drillingconditions, abnormal formation pressures, open fractures, thepermeability of the formation, and formation pressure.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a drill string that is suspended in aborehole together with a logging-while-drilling apparatus that derivesstreaming potential measurements in accordance with the presentinvention.

FIG. 2 is a cross-sectional view of the bottom-hole assembly of thedrill string, which embodies an exemplary embodiment of alogging-while-drilling apparatus in accordance with the presentinvention.

FIG. 3 is schematic representing a forward model of an exemplary bottomhole assembly that embodies a logging-while-drilling apparatus inaccordance with the present invention.

FIG. 4 is a plot of an oscillating streaming potential signal measuredat electrode E1 and an over-balanced pressure for the model of FIG. 3.

FIG. 5 is a section of the plot of FIG. 4 in an expanded time scale.

FIG. 6 is a schematic diagram of the signal processing operations thatderive the various components of the streaming potential signalsmeasured by the logging-while-drilling apparatus of FIGS. 1, 2 and 3.

FIG. 7A is a plot of pressure at a point near the borehole surface.

FIG. 7B is a plot of pressure at an interior point of the formation awayfrom the borehole surface.

FIG. 8 is a plot of the DC, in-phase and out-of-phase components of thestreaming potential signal measured by the electrode E1 in conjunctionwith an oscillating over-balanced pressure, which is generated by themodel of FIG. 3.

FIG. 9 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3 and E4 with the isolation jointlocated 3 feet behind the drill bit, which is generated by the model ofFIG. 3.

FIG. 10 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3 and E4 with the isolation jointlocated 5 feet behind the drill bit, which is generated by the model ofFIG. 3.

FIG. 11 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3 and E4 with the isolation jointlocated 12 feet behind the drill bit, which is generated by the model ofFIG. 3.

FIG. 12 is a plot of the in-phase and out-of-phase components of thestreaming potential signal measured by the electrodes E1, E2, E3, E4with the isolation joint located 3 feet behind the drill bit, which isgenerated by the model of FIG. 3.

FIG. 13 is a plot of the in-phase and out-of-phase components of thestreaming potential signal measured by the electrodes E1, E2, E3, E4with the isolation joint located 5 feet behind the drill bit, which isgenerated by the model of FIG. 3.

FIG. 14 is a plot of the in-phase and out-of-phase components of thestreaming potential signal measured by the electrodes E1, E2, E3, E4with the isolation joint located 12 feet behind the drill bit, which isgenerated by the model of FIG. 3.

FIG. 15 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3, E4 without an isolation joint,which is generated by the model of FIG. 3.

FIG. 16 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3 and E4 in conjunction with anunder-balanced drilling condition, which is generated by the model ofFIG. 3.

FIG. 17 is a plot of the in-phase and out-of-phase components of thestreaming potential signal measured by the electrodes E1, E2, E3 and E4in conjunction with an under-balanced drilling condition, which isgenerated by the model of FIG. 3.

FIG. 18 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3 and E4 in conjunction withdrilling through an open fracture that is closed later by mudcake, whichis generated by the model of FIG. 3.

FIG. 19 is a plot of the in-phase and out-of-phase components of thestreaming potential signal measured by the electrodes E1, E2, E3 and E4in conjunction with drilling through an open fracture that is closedlater by mudcake, which is generated by the model of FIG. 3.

FIG. 20 is a plot of the DC component of the streaming potential signalmeasured by the electrodes E1, E2, E3 and E4 in conjunction withdrilling through an open fracture that is not closed by mudcake, whichis generated by the model of FIG. 3.

FIG. 21 is a plot of the in-phase and out-of-phase components of thestreaming potential signal measured by the electrodes E1, E2, E3 and E4in conjunction with drilling through an open fracture that is not closedby mudcake, which is generated by the model of FIG. 3.

FIG. 22 is a plot of the DC component of the streaming potential signalmeasured by the electrode E1 over a set of varying radial permeabilitiesand a vertical permeability of 100 mD, which is generated by the modelof FIG. 3.

FIG. 23 is a plot of the DC component of the streaming potential signalmeasured by the electrode E1 over a set of varying verticalpermeabilities and a radial permeability of 100 mD, which is generatedby the model of FIG. 3.

FIG. 24 is a plot of the DC component of the streaming potential signalmeasured by the electrode E1 over a set of varying verticalpermeabilities and a radial permeability of 4 mD, which is generated bythe model of FIG. 3.

FIG. 25 is a plot of discrete log time derivatives of the DC componentof the streaming potential signal measured by the electrode E1 over aset of varying radial permeabilities and a vertical permeability of 100mD, which is generated by the model of FIG. 3.

FIG. 26 is a plot of discrete log time derivatives of the DC componentof the streaming potential signal measured by the electrode E1 over aset of varying vertical permeabilities and a radial permeability of 100mD, which is generated by the model of FIG. 3.

FIG. 27 is a plot of discrete log time derivatives of the DC componentof the streaming potential signal measured by the electrode E1 over aset of varying vertical permeabilities and a radial permeability of 4mD, which is generated by the model of FIG. 3.

FIG. 28 is a plot of the ratio of the out-of-phase component to thein-phase component of the streaming potential signal measured by theelectrode E1 over a set of varying radial permeabilities and a verticalpermeability of 100 mD, which is generated by the model of FIG. 3.

FIG. 29 is a plot of the ratio of the out-of-phase component to thein-phase component of the streaming potential signal measured by theelectrode E1 over a set of varying vertical permeabilities and a radialpermeability of 100 mD, which is generated by the model of FIG. 3.

FIG. 30 is a plot of the ratio of the out-of-phase component to thein-phase component of the streaming potential signal measured by theelectrode E1 over a set of varying vertical permeabilities and a radialpermeability of 4 mD, which is generated by the model of FIG. 3.

FIG. 31 is a plot of the DC component of the streaming potential signalmeasured by the electrode E1 in conjunction with a set of over-balancedpressure values, which is generated by the model of FIG. 3.

FIG. 32 is a diagram illustrating curve-fitting operations that derivean estimate of formation pressure while drilling based upon themagnitude of streaming potential measurements over a set of varyingover-balancing pressures.

FIG. 33 is a schematic diagram of a logging-while drilling tool and adrill bit.

FIG. 34 is a schematic diagram of a drill bit and a logging-whiledrilling tool with all electrodes insulated from the rest of the tool.

FIGS. 35A and 35B are schematic diagrams of multiple electrodesdistributed about the circumference of a drill collar.

DETAILED DESCRIPTION

Prior to turning to the Figures, some theoretical considerationsgoverning the physics of the invention are useful. In reservoir rocksthere exists a thin charged double layer at the interface between theporous rock matrix and water in the porous rock matrix. In typicalconditions, the matrix surface is negatively charged, and the water ispositively charged. When water moves under a pressure gradient ∇p, anelectrical current i_(e) is created with the water current. Theelectrical current is proportional to the water current, which isproportional to the pressure gradient:i_(e)=L ∇p,   (1)

where L is a coupling constant which is a property of thefluid-saturated rock.

Pressure transients are created in the formation by many differentoperations that occur over the lifetime of a well such as drilling, mudinvasion, cementing, water and acid injection, fracturing, and oil andgas production. Pressure transient testing is an established techniqueto determine reservoir properties such as permeability, reservoir size,and communication between different zones and between different wells.As is set forth below, streaming potential transients associated withthe pressure transients can also be used to determine these properties.

The modeling of the reservoir pressure p can be carried out withmultiphase flow models. For the modeling of the streaming potential, itis useful to start with the diffusion equation of a single-phase flow:$\begin{matrix}{{{{\nabla{\cdot \frac{k}{\mu}}}{\nabla p}} = {\phi\quad c\frac{\partial}{\partial t}p}},} & (2)\end{matrix}$where k is the permeability, μ is the viscosity, φ is the porosity, andc is the fluid compressibility.

From the modeled pressure field p, the streaming potential V can becalculated by solving the Poisson Equation:−∇·σ∇V=∇·L ∇p,   (3)

where σ is the electrical conductivity.

From Eq. (2) it follows that the time Δt for a pressure transient andthe associated streaming potential transient created at the boreholesurface to diffuse through a distance Δx into the formation is given by:$\begin{matrix}{\Delta\quad{\left. t \right.\sim\frac{\phi\quad c\quad\mu}{k}}{\left( {\Delta\quad x} \right)^{2}.}} & (4)\end{matrix}$

The early time pressure and streaming potential transients are sensitivemainly to reservoir properties near the borehole, and the late timetransients are sensitive to reservoir properties both near the boreholeand farther away from the borehole. By interpreting the measuredtransients in a time ordered fashion, reservoir properties at differentdistances to the borehole can be determined. The interpretation ofpressure transients in this time ordered fashion is an established art.For example, early time pressure transients are used to determine damageto permeabilities of “skin”, and late time pressure transients are usedto determine reservoir boundaries. As is set forth below, streamingpotential transients associated with the pressure transients can be usedto determine these properties.

The applications are much more limited if the steady state values of thestreaming potentials are the only measurements available. At a steadystate, equation (2) becomes $\begin{matrix}{{{\nabla{\cdot \frac{k}{\mu}}}{\nabla p}} = 0.} & (5)\end{matrix}$The pressure drop Δp across a depth interval Δx is then proportional to$\begin{matrix}{{\Delta\quad p} \propto {\frac{\mu}{k}\Delta\quad{x.}}} & (6)\end{matrix}$The drop in the streaming potential ΔV is related to Δp by$\begin{matrix}{{{\Delta\quad V} = {{- \frac{L}{\sigma}}\Delta\quad p}},} & (7)\end{matrix}$which is proportional to $\begin{matrix}{{\Delta\quad V} \propto {\frac{L\quad\mu}{\sigma\quad k}\Delta\quad{x.}}} & (8)\end{matrix}$

The steady state streaming potential can only give information on theaverage value of a reservoir property and as a result is dominated byintervals with high values of (Lμ)/(σk). It is believed that in thepresence of a mudcake, the steady state streaming potential is dominatedby the mudcake and is insensitive to reservoir properties. Thepermeability of the mudcake is extremely low, and the steady statepressure drop mainly exists across the mudcake.

While in principle it is possible to determine reservoir properties atall distances to the borehole (i.e., radially from the borehole) byinterpreting the transients in a time ordered fashion, the criticalquestion in practice is whether the measurements can be made withsufficient quality: accuracy, spatial resolution, and stability overlong time. It is difficult to get pressure transient data with highspatial resolution as the borehole is essentially an isobaric region. Apressure sensor placed inside the borehole cannot give detailedinformation on the pressure transients inside the formation if theformation is heterogeneous. To do so, it would be necessary to segmentthe borehole into hydraulically isolated zones, a difficult andexpensive task to perform. On the other hand, the borehole is not anequipotential surface for electric current flow. Thus, streamingpotential transients may be measured by an array of electrodes placedinside the borehole and electrically isolated (i.e., insulated) one fromthe other and can provide equivalent information to that ofhydraulically isolated zone pressure transient testing because thestreaming potential is determined by the pressure gradient. In fact, byutilizing an array of isolated streaming potential electrodes, thestreaming potential can be measured with a higher spatial resolutionthan hydraulically isolated zone pressure transient testing.

Formation properties vary from place to place (inhomogeneity) and mayalso vary with direction at a given place (anisotropy). Consequently,the streaming potential at a particular depth may vary around thecircumference of the borehole. For example, a fracture that crosses theborehole at an oblique angle can produce significant changes instreaming potential with azimuth. Another example is permeabilityanisotropy, which modifies the flow distribution resulting from apressure transient. Since the flow distribution affects the streamingpotential, permeability anisotropy can also produce azimuthal variationsin streaming potential. Thus, azimuthal measurements of streamingpotential may be interpreted to determine inhomogeneity and anisotropyof formation properties.

Given the theoretical understandings above, according to one aspect ofthe invention, an apparatus and method for measuring streamingpotentials while drilling a borehole is provided. In particular, duringdrilling, a pressure difference between the formation and the boreholecreates mud invasion and pressure transients, and thus, streamingpotential transients. In wells drilled with an oil-based mud, astreaming potential will exist if the mud contains a water fraction.

Turning now to FIG. 1, a schematic illustration of a borehole 10 drilledinto a formation 12 by a rotary drilling apparatus that employs awhile-drilling streaming potential measurement tool in accordance withthe present invention. The drilling apparatus includes a drill string 14composed of a number of interconnected tubular drill collar sections(including the six shown as 15A, 15B, 15C, 15D, 15E, 15F) supporting attheir lower end a drill collar 16 terminated by a drill bit 17. At thesurface, the drill string 14 is supported and rotated by standardapparatus (not shown), thereby rotating the drill bit 17 to advance thedepth of the borehole 10.

A recirculating flow of drilling fluid or mud is utilized to lubricatethe drill bit 17 and to convey drill tailings and debris to the surface18. Accordingly, the drilling fluid is pumped down the borehole 10 andflows through the interior of the drill string 14 (as indicated by arrow19), and then exits via ports (not shown) in the drill bit 17. Thedrilling fluid exiting the drill bit 17 circulates upward (as indicatedby arrows 20) in the region between the outside of the drill string 14and the periphery 21 of the borehole, which is commonly referred to asthe annulus.

The bottom portion of the drill string 14, including the drill bit 17,the drill collar 16 and at least one tubular drill collar sectionconnected the drill collar 16 (e.g. drill collar section 15F), isreferred to as a bottom hole assembly 100. In accordance with thepresent invention, the bottom hole assembly 100 includes capabilities ofmeasuring streaming potential while-drilling as described below in moredetail. The while-drilling measurements are observed in the borehole 10with the bottom hole assembly 100 located in the borehole duringdrilling, pausing, tripping or other operations.

As shown in the cross-section of FIG. 2, the bottom hole assembly 100includes a series of interconnected elements including the tubular drillcollar section 15F, the drill collar 16 and the drill bit 17. The drillcollar section 15F includes a metal body 101 and three external ringelectrodes 102-1, 102-2, 102-3 that are spaced apart longitudinally andmounted on a suitable insulating medium (e.g., insulating pads 103-1,103-2, 103-3) that electrically insulates the electrodes from the metalbody 101. The drill collar section 15F also includes a pressure sensor104 mounted thereon for measuring the downhole pressure.

The drill bit 17 is interconnected to the bottom of the drill collar 16by a threaded coupling 105. The top of the drill collar 16 isinterconnected to the bottom of the drill collar section 15F by aninsulation joint 106 that electrically insulates the metal body of thedrill collar section 15F from the drill collar 16 and the drill bit 17.The insulation joint 106 allows the metal body of the drill collar 16and drill bit 17, collectively, to be used as a measuring electrode forstreaming potential measurements and also allows the metal body 101 ofthe drill collar section to be used as a voltage reference electrode forstreaming potential measurements as described below in more detail. Inthe preferred embodiment, one of the electrodes (102-1) is locatedadjacent to (or on) the insulating joint 106. In this configuration, theelectric streaming current created by the spurt loss at the drill bit 17is forced to flow further out in the formation 12 and into the drillstring above the isolation joint 106. With the electrode 102-1 placedadjacent to (or on) the isolation joint 106, it will have a strongsignal even if it is a good distance behind the drill bit 17.

An annular chassis 107 fits within the drill collar section 15F and thedrill collar 16. The annular chassis does not provide an electricalshort circuit between the drill collar section 15F and the drill collar16. Preferably, the annular chassis ends at the isolation joint and aninsulated wire connects electronics inside the annular chassis to thedrill collar 16. Alternatively, if a portion of the annular chassisextends below the isolation joint, it is made of a non-conductivematerial (for example, made of fiberglass epoxy). The annular chassis107 houses wiring that is electrically coupled via insulatedfeed-throughs (not shown) to the electrodes 102-1, 102-2, 102-3, thepressure sensor 104, the metal body of the drill collar/drill bit (whichis used as a measuring electrode for streaming potential measurements),and the metal body 101 of the drill collar section 15F (which is used asa reference electrode for streaming potential measurements). Thedrilling fluid flows through the center of the annular chassis 107 asshown by the arows 108. The annular chassis 107 also preferably includesinterface electronics and telemetry electronics which interface to anMWD mud pulse telemetry system located in a separate drill collar. TheMWD mud pulse telemetry system generates oscillating pressure waves thatpropagate upwards inside the drill string, and which are detected by apressure sensor mounted on the drilling rig. The mud pulse telemetrysystem encodes the downhole measurements, which are decoded by thesurface-located data processing equipment (e.g., a processor and,associated data storage). The data processing equipment receives datasignals representative of the streaming potentials measured by theelectrodes as well data signals representative of the pressure measuredby the pressure sensors. Such data signals are analyzed to obtain answerproducts as discussed below.

The streaming potential values measured by the electrodes are passivevoltage readings, which can be made in a highly resistive borehole byusing high impedance electronics. In wells drilled with oil-based mud,the electrodes need to be as large as possible and placed as close aspossible to the formation to reduce electrode impedance.

It will be appreciated by those skilled in the art that in order toproperly analyze the data signals derived from the electrodes and/orpressure sensor of the bottom hole assembly 100, a model of mudcakebuilt up during drilling should be included in the forward model.Drilling muds are designed to prevent significant loss of borehole fluidby forming a nearly impermeable barrier—mudcake—on the borehole wall.The mudcake consists of clays and fine particles that are left behindwhen mud invades a permeable formation. Accurate models of mudcakeformation and mud filtration such as disclosed in E. J. Fordham and H.K. J. Ladva, “Crossflow Filtration of Bentonite Suspensions”,Physico-Chemical Hydrodynamics, 11(4), 411-439 (1989) can be utilized.With an appropriate model, the streaming potential information derivedfrom the electrodes and/or pressure sensor of the bottom hole assembly100 can yield various answer products that are applicable to a widevariety of applications such as drilling safety and formationevaluation. For example, the applications within drilling safety includethe early detection of under-balanced condition in over-pressured zones,early detection of fluid loss through fractures and faults, and theestimation of formation pressure. In other examples, the applicationswithin formation evaluation include the estimation of formationpermeability and the evaluation of fractures.

An exemplary model is illustrated in the schematic diagram of FIG. 3. Inthis model, the electrode 102-1 (referred to below as electrode “E2”) islocated at the mid-point of a 0.5 ft long isolation joint 106 with theelectrodes 102-2 and 102-3 (referred to below as electrodes “E3” and“E4”, respectively) located in the midpoint of 1 ft long surfaceinsulating sections 102-2, 102-3, which are offset by 1 ft and 4 ft,respectively, from the top of the insulation joint 106. Three differentlengths (3 ft., 5 ft and 12 ft) of the drill collar 16 are used. Thedrill collar/drill bit is referred below as electrode E1. The metal bodyof the drill collar section 15F defines a reference electrode. Theborehole is a vertical well. The conductivity of the formation is 1 S/m,the conductivity of the annulus is 10 S/m, and the conductivity of thedrill collar 16 and the drill bit 17 is 10⁷ S/m. The mud pump and themud pulse telemetry cause the bottom-hole pressure to oscillate. The mudpulse telemetry system can create pressure pulses of a few hundred psi(pounds per square inch) at frequencies from below 1 Hz to above 20 Hz.The difference between the bottom hole pressure and the formation istaken to be$p = {p_{0} + {p_{1}{\sin\left( {2\pi\frac{t}{T}} \right)}}}$where the initial pressure p₀ is assumed to be 500 psi as an example ofover-balanced drilling, and is assumed to be −500 psi as an example ofunder-balanced drilling. The oscillating pressure from the mud pulsetelemetry system is p₁, which is assumed to be is 35 psi, and the periodof the mud pulse signal is T, which is assumed to be 1 second. In thefollowing examples, the rate of penetration of the drill bit is assumedto be constant at 33 ft/hr till the drilling stops. The drill bit andthe drill collars located above it are assumed to be in an impermeableformation (12A of FIG. 1) until time t=0, when the drill bit cuts into apermeable formation (12B of FIG. 1). For times t<0, there is nostreaming potential because there is no fluid flowing into, or out of,the impermeable formation. For times t>0, the fluid in the boreholeflows into the permeable formation (over-balanced situation), or fluidfrom the permeable formation flows into the borehole (under-balancedsituation). The amount of permeable formation exposed to the borehole isgiven by the rate of penetration multiplied by the time, t. At timet=800 seconds, the drilling stops with the drill bit having penetrated7.3 ft into the permeable formation; but the mud pump and the mud-pulsetelemetry continue to operate. In the over-balanced case, a mudcakecontinues to form and reduces the flow of fluid into the formation. Inthe under-balanced case, the fluid continues to flow out of theformation since mudcake cannot form.

An oscillating streaming potential signal may be derived from adifferential voltage measurement (block 601 of FIG. 6) between thereference electrode and one of the four electrodes (e.g., E1, E2, E3, orE4), which is herein referred to as the “measuring electrode”. Forexample, an oscillating streaming potential signal derived from thedrill collar/drill bit (electrode E1) and an overbalancing pressure isshown in the graph of FIG. 4. A section of FIG. 4 is shown in expandedtime scale in FIG. 5. Between time t=0 and t=800 seconds, the drill bitcuts into the permeable formation (12B). At t=800 seconds, electrodesE1, E2, and E3 are in the permeable formation, while E4 remains in theimpermeable formation.

In the over-balanced case, mud filtrate invades the permeable formation,producing a streaming potential. Most of the invasion occurs at thecutting face of the drill bit, which continuously removes mudcake.Behind the drill bit's cutting face, mudcake forms on the borehole walland inhibits further invasion. When the drilling stops at t=800 seconds,the invasion also stops, and the streaming potential rapidly decreases.

In the under-balanced case, formation fluids flow into the wellborealong the entire length of the borehole in the permeable formation, aswell as at the cutting face of the bit. Because mudcake cannot form, thestreaming potential does not decrease once the drilling stops.

Alternatively, separate DC and AC components of the oscillatingstreaming potential signal can be acquired as shown in FIG. 6. The DCcomponent is acquired in block 602 by averaging the oscillatingstreaming potential signal output by the differential voltagemeasurement of block 601 over time. The AC component is acquired inblock 603 by filtering out the DC component (and other unwantedcomponents) of the oscillating streaming potential signal output by thedifferential voltage measurement of block 601. Furthermore, the in-phasecomponent and the out-of-phase component of the streaming potentialsignal (hereinafter referred to as AC components) can be acquired inblock 605 by synchronous detection of the AC component of the streamingpotential signal (output by block 603) with the AC component of thebottom hole pressure measured by the pressure sensor (output by block604). The AC component of the bottom hole pressure is acquired in block604 by filtering out the DC component (and other unwanted components) ofthe pressure signal output by the pressure. Importantly, the AC of theoscillating streaming potential signal are less subject to noisecontamination than the DC components of the oscillating streamingpotential signal.

The origin of the out-of-phase component of the streaming potentialsignal can be understood in the following way. The oscillating bottomhole pressure will diffuse into the formation. At an interior point inthe formation the pressure will have a phase difference with the bottomhole pressure. The phase difference depends on the distance to theborehole and on the formation permeability and the fluid properties. Thestreaming current at an interior point will be in phase with thepressure gradient at that point and have a phase difference with thebottom hole pressure. The streaming current at each interior point givesa contribution to the streaming potential at the measuring electrode;the contribution is in phase with the pressure gradient at the interiorpoint. The streaming potential at the measuring electrode is theintegral of the contributions from all points in the formation.Therefore, there will exist an out-of-phase component of the streamingpotential signal.

Since the pressure is governed by the diffusion equation rather than thewave equation, the response of the pressure at some distance away fromthe source becomes diffuse. At large distance away, the detailedinformation about the source, both spatially and temporally, will belost in the pressure. Therefore, the oscillatory part of the pressureshould diminish with increasing distance to the source of the pressuredisturbance. The pressure at a point near the borehole surface isplotted in FIG. 7A, and the corresponding pressure at an interior pointof the formation away from the borehole surface is plotted in FIG. 7B.It can be seen the amplitude of the oscillatory part as a percentage ofthe DC part does decrease significantly as one moves away from thesource. Consequently, the AC component of the streaming potential hasshallower depths of investigation than the DC component.

The DC, the AC in-phase component, and the AC out-of-phase components ofa streaming potential signal measured by the electrode E1 are shown inFIG. 8. The in-phase and out-of-phase components are plotted atdifferent scales from the DC component, with the AC componentsmultiplied by a factor of 10 for clarity. These AC components saturateearlier than the DC component and decline more rapidly when the drillingstops. These properties reflect the short-range nature of the ACcomponents of the streaming potential signal.

The phase difference between the bottom hole pressure and the streamingpotential depends on formation permeability. The potential use of thephase difference in the determination of formation permeability will bediscussed in a later section.

The streaming potential signals measured by the four electrodes E1, E2,E3 and E4 i are dependent upon the positions of the electrodes along thedrill string as well as the position of the isolation joint 16 along thedrill string. The streaming potential signals at the four electrodes areshown for over-balanced conditions in FIGS. 9-15. The isolation joint106 is located different distances from the drill bit 17 in thesefigures.

The DC signal components are shown in FIGS. 9, 10 and 11, respectively.Increasing spacing between the drill bit 17 and the isolation joint 106from 3 ft to 12 ft does not cause a large drop in the DC signalamplitude. The signal amplitude for electrode E1 is approximately 38 mVfor a 3 ft spacing, and approximately 23 mV for a 12 ft spacing betweenthe bit and the isolation joint. The signal amplitude of electrode E2 isapproximately 19 mV for the 3 ft spacing, and 12 mV for the 12 ftspacing. Also note that electrode E2 is in the permeable bed for the 3ft spacing, but is in the impermeable bed for the 12 ft spacing. Thisslow decrease in the DC signal amplitudes occurs because the drill bit17 and the drill collar 16 form an equipotential surface and passivelyfocus the streaming potential electric current away from the borehole.The isolation joint 106 forces the streaming electrical current toreturn to the drill collar section 15F behind the isolation joint 106.This also explains why the DC amplitudes decrease slowly from E1 to E2.Once the streaming electric current flows beyond the isolation joint106, it quickly returns to the drill collar 15F. This explains why theamplitude at electrode E3 is much smaller than at E2.

The AC signal components are shown in FIGS. 12, 13, and 14,respectively. The amplitudes of the AC components decrease slowly withincreasing distance between the drill bit and the isolation joint

The DC signal components calculated without the isolation joint 106 areshown in FIG. 15. Comparing FIGS. 15 and 9, the amplitudes without theisolation joint are 40 times smaller than with an isolation joint. Thus,an isolation joint is preferably included in the tool design in order toprovide good signal strength.

FIGS. 9-15 show that accurate measurements of streaming potentialsignals can be obtained preferably with an isolation joint located inthe drill string. The isolation joint 106 need not be locatedimmediately behind the drill bit 17. Note that when the distance betweenthe isolation joint 106 and the drill bit 17 increased from 3 ft to 12ft, the magnitude of the streaming potential decreased only by a factorof 2. Thus, it is contemplated that the isolation joint 106 may beplaced 20 ft to 30 ft behind the drill bit 17 if desired. This allows adirectional drilling system to be located immediately above the drillbit, and the isolation joint to be placed above the directional drillingsystem. This is an important practical consideration since most offshorewells require a directional drilling system immediately above the drillbit.

In accordance with one aspect of the present invention, the streamingpotential signals measured by one or more of the four electrodes E1, E2,E3, E4 of the assembly 100 can be used to detect under-balanced drillingconditions, which is critical for drilling safety. Normally, anover-balanced condition is maintained during drilling. If a formationwith unexpectedly high pressure is encountered such that anunder-balanced condition exists, the driller must take immediate actionto prevent a “blow-out”. A “blow-out” can result in the loss of thewell, environmental damage, and potentially the loss of life. FIG. 16shows the DC signal components of the streaming potential signals withthe isolation joint 106 located 3 ft behind the drill bit 17. FIG. 17shows the AC signal components. The detection of under-balanced drillingconditions can be simply derived from the DC signals by detecting a signreversal in the DC signals. From FIGS. 9 and 16, the sign of a DC signalis negative for over-balanced drilling conditions and positive forunder-balanced drilling conditions. The DC signal will go from negativeto positive when a transition from over-balanced to under-balanceddrilling conditions occurs.

Alternatively, the AC signal components can be used to detectunder-balanced drilling conditions. Comparing FIGS. 12 and 17, there isno sign reversal in the AC components when a transition fromover-balanced to under-balanced drilling conditions occurs. However, thetime duration of the rising period of the streaming potential afterentering the permeable zone is very different in the two figures. In theover-balanced case, mudcake rapidly forms on the borehole wall above thedrill bit and reduces the amount mud filtrate invasion behind the drillbit's cutting face. Hence, significant filtration only occurs at thecutting face. Increasing the borehole length in the permeable formation12B does not increase the rate of filtration. In the under-balancedsituation, mudcake cannot form. Hence the amount of mud filtrateincreases as the length of the borehole in the permeable formationincreases. Moreover, when the drilling is stopped at t=800 seconds, theAC signal components rapidly decline for over-balanced drillingconditions (FIG. 12), even though drilling fluid continues to circulate,but remain steady for under-balanced drilling conditions (FIG. 17).Again, the rapid build-up of mudcake in the over-balanced case explainswhy the streaming potential decreases rapidly at t=800 seconds. Whereasin the under-balanced case, the fluid continues to flow from theformation into the borehole. Finally, the AC signal components atelectrode E3 remain very small for over-balanced drilling conditions(FIG. 12), but increase significantly (e.g., at t=600) forunder-balanced drilling conditions (FIG. 17). In accordance with thepresent invention, one or more of these properties can be used asdetection criteria for under-balanced drilling conditions.

In accordance with another aspect of the present invention, thestreaming potential signals measured by one or more of the fourelectrodes E1, E2, E3, E4 of the assembly 100 can be used to detect openfractures, which is critical for drilling safety. In particular, theremay be sudden fluid loss from natural or induced fractures duringdrilling, resulting in lost circulation and potentially dangerously lowpressures in the borehole. In that case, the streaming potential willrapidly increase as fluids rush into the formation through the openfractures. This fluid loss will not be noticed at the surface until muchlater. FIG. 18 shows the DC signal components with the isolation joint106 located 3 ft behind the drill bit 17 in conjunction with an openfracture that is later sealed by mudcake. FIG. 19 shows the AC signalcomponents. FIG. 20 shows the DC signal components if the fracture isnot sealed by mudcake and FIG. 21 shows the AC signal components. Thefracture is a thin zone of high permeability, and it is penetrated bythe drill bit at t=320 seconds. As is evident from FIGS. 18-21, fluidflow increases as the fracture is drilled through, which results inlarge DC and AC streaming potential signals at electrodes E1 and E2. TheDC components are 40 times larger for electrode E1 with the fracturethan with the permeable formation (FIG. 9). Similarly, the AC signalcomponents are significantly larger for electrode E1 with the fracturethan with the permeable formation (FIG. 12). When the fracture is latersealed by mudcake (at t=380 seconds in FIGS. 18 and 19), the DCcomponents and the AC components revert back to their steady statevalues after the drill bit moves beyond the fracture. When the factureis not sealed by mudcake (FIGS. 20 and 21), there is no decrease whenthe drill bit leaves the fracture. In accordance with the presentinvention, these properties can be used as detection criteria for anopen fracture, which is also critical for drilling safety.

In accordance with yet another aspect of the present invention, thestreaming potential signals measured by one or more of the fourelectrodes E1, E2, E3, E4 of the assembly 100 can be used tocharacterize the radial permeability and vertical permeability of theformation. After drilling is stopped, mudcake quickly forms to stopfluid loss in an over-balanced situation. The streaming potentialsignals decrease with time as the pressure gradient diffuses into theformation. The decay is fast for formations of high permeability andslow for formations of low permeability. FIG. 22 shows the DC componentmeasured by electrode E1 for five different values of radialpermeability with a vertical permeability of 100 mD. It can be seen thatthe decay rate is quite sensitive to radial permeability. FIG. 23 showsthe DC component measured by electrode E1 for five different values ofvertical permeability with a radial permeability of 100 mD. FIG. 24shows the DC component measured by the electrode E1 for five differentvalues of vertical permeability with the radial permeability fixed at 4mD. For better visualization of the sensitivity of the decay rate topermeability, the discrete log time derivatives (the difference betweenthe DC components at two time sample points divided by the DC componentat the mid time sample point) are shown in FIGS. 25-27, wherein the timeinterval is a short interval of around 800 seconds when the drillingstops. The sensitivity of the DC component to the radial permeability ofthe formation is clear from the relatively large separations between thecurves of FIG. 25. The insensitivity of the DC component to verticalpermeability with the radial permeability fixed at 100 mD is clear fromthe relatively small separations between the curves of FIG. 26. And theinsensitivity of the DC component to vertical permeability with theradial permeability fixed at 4 mD is clear from the relatively smallseparations between the curves of FIG. 27. From these properties, it canbe concluded that after the drilling stops and mudcake builds up, therelaxation of the excess pressure near the borehole is primarily in aradial direction.

FIG. 28 depicts the ratio of the out-of-phase component to the in-phasecomponent (i.e. the tangent of the phase angle) of the AC streamingpotential signal measured by electrode E1 for five different values ofradial permeability with the vertical permeability fixed at 100 mD. FIG.29 depicts the tangent of the phase angle of the streaming potentialsignal measured by electrode E1 for five different values of verticalpermeability with the radial permeability fixed at 100 mD. And FIG. 30depicts the tangent of the phase angle of the streaming potential signalmeasured by electrode E1 for five different values of verticalpermeability with the radial permeability fixed at 4 mD. It can be seenthat the tangent of the phase angle of the streaming potential signal issensitive to both the radial permeability and the vertical permeability.In all three FIGS. 28-30, the absolute value of the phase angle tangentdecreases as the permeability increases. In the limit of infinitepermeability (or zero compressibility) the pressure response inside theformation to the borehole pressure variations is instantaneous. Sourcesof streaming currents inside the formation are always in phase with theborehole pressure. The phase angle approaches zero as the permeabilityapproaches infinity.

Since the decay rates and the phase angle tangents have differentsensitivities to the radial permeability and to the verticalpermeability, it is possible to estimate both the radial permeabilityand the vertical permeability with the combined DC and AC components. Byminimizing the difference between the measured and calculated values,both radial permeability and vertical permeability are estimated.

If the DC signal component of the streaming potential signal isunavailable, it is possible to estimate the permeability of theformation with the AC components alone. In this analysis, an assumptionis made as the ratio between the radial permeability and the verticalpermeability.

In accordance with yet another aspect of the present invention, thestreaming potential signals measured by one or more of the fourelectrodes E1, E2, E3, E4 of the assembly 100 can be used to estimateformation pressure. FIG. 31 shows the DC component of the streamingpotential signal measured at electrode E1 for three differentover-balancing pressures (500 psi, 400 psi, and 300 psi). The boreholepressure can be varied by controlling the rate of the mud pumps, byvarying the weight of the drilling mud, or by tripping the drill stringinto (or out) of the borehole. It is evident from FIG. 31 that the DCstreaming potential signals are proportional to the over-balancepressure. Therefore, the formation pressure can be estimated bymeasuring the DC components of the streaming potential at a plurality ofdifferent borehole pressures. The magnitudes of the streaming potentialfor the different over-balancing pressures are fit to a straight lineaccording to Darcy's law. This straight line is extrapolated todetermine the formation pressure at zero streaming potential. Thispressure (the pressure at zero streaming potential) is the formationpressure. The estimation method is shown schematically in FIG. 32. Theestimation of formation pressure while drilling is extremely importantfor drilling safety.

In accordance with yet another aspect of the present invention, thestreaming potential signals measured by one or more of the fourelectrodes E1, E2, E3, E4 of the assembly 100 can also be used for theearly detection of abnormal formation pressures. For example, if theformation pressure becomes higher than the borehole pressure, the signof the DC component of the streaming potential signals will reverse.This reversal of sign will be observable before sufficient fluid hasflowed into the borehole for the pressure change to be directlyobservable. The build-up of the flow reversal may happen over a shortbut finite period of time as the abnormal pressure zone is beingdrilled. Any reversal of flow will be immediately observable in the DCcomponent of the streaming potential measurements. Therefore, the DCcomponent streaming potential measurements have value in the earlydetection of abnormal formation pressure. Because the streamingmeasurement is made very close to the drill bit, this provides theearliest possible warning of an over-pressured formation.

A preferred embodiment of an LWD tool 100′ designed to measure thestreaming potential is shown in FIG. 33. The drill bit 17′ is attachedto the drill collar section 16′ of the LWD tool 100′ by a coupling 105′.The top of the drill collar 16′ is interconnected to the bottom of drillcollar section 15F′ by an insulation joint 106′ that electricallyinsulates the metal body of the drill collar section 15F′ from the drillcollar 16′ and the drill bit 17′. The insulation joint 106′ allows themetal body of the drill collar 16′ and drill bit 17′, collectively, tobe used as a measuring electrode for streaming potential measurementsand also allows the metal body 101′ of the drill collar section 15F′ tobe used as a voltage reference electrode for streaming potentialmeasurements as described herein. The drilling mud 108′ passes thoughthe interior of the LWD tool and the drill bit 17′ to lubricate thedrill bit 17′ and to remove cuttings. A pressure sensor 104′ measuresthe borehole pressure. The insulated joint 106′ can consist of a shopconnection where the pin has a thin ceramic coating, approximately 0.010to 0.020 inches thick. The ceramic-coated pin connection is screwed intoa mating box connection, and the assembled shop connection is injectedwith epoxy. This shop connection is permanent and not broken after thetool is manufactured. This assembly can provide a high degree ofelectrical insulation between the drill collar 16′ and the collarsection 15F′.

A layer of insulation 103-1′ covers the exterior of the insulated joint106′, while another layer of insulation 112 covers the inside. Theseexternal and internal layers of insulation can be made of fiberglassand/or rubber, for example. Their purpose is to increase the electricalresistance between the lower electrode E1 (drill collar 16′ and drillbit 17′) and the drill collar 15F, which would otherwise be reduced whenconductive drilling mud is present. The external layer of insulation103-1′ also provides electrical insulation around the E2 electrode102-1′ to increase the signal strength. The electrodes 102-1′ (E2),102-2′ (E3) and 102-3′ (E4) can be metal rings embedded in theinsulating layers 103-1′, 103-2′, and 103-3′, respectively. Each ofthese three electrodes is attached to the measurement electronics bypressure bulkheads and wires (not shown). These external insulatinglayers and electrodes can be mounted flush with the exterior of thedrill collar or slightly recessed to prevent damage. Wearbands with aslightly larger diameter than the drill collar can also be added toprevent damage to the electrodes.

Electrode E1 (drill collar 16′ and drill bit 17′) is connected to themeasurement electronics that are housed in the annular chassis 107′ byan internal electrical extender 110 that plugs into an electrical socket111. The electrical socket 111 is directly attached to a top interiorportion of the drill collar 16′ to provide electrical connection to theE1 electrode (drill collar 16′ and drill bit 17′). The electricalextender 110 consists of a wire or medal rod that mates with theelectrical socket 111 when the shop connection is made-up. An insulatinglayer surrounds the electrical extender 110, and serves the same purposeas the internal insulating layer 112.

Electronics chassis 107′ preferably contains the measurementelectronics, a processor and memory, a clock, communication electronics,and may also contain a battery. The annular chassis 107′ is preferablymade of metal and provides an airtight chamber for the electronics. Theannular chassis 107′ may be held inside the drill collar section 15F′with jam-nut 113 which threads into the collar section 15F′. Anelectrical extender 114 on the upper connection of the LWD tool providesan electrical connection to an MWD tool (not shown), which communicateswith the drilling rig via mud pulse telemetry or electromagnetictelemetry.

Another embodiment of an LWD tool 100″ designed to measure the streamingpotential is shown in FIG. 34. This embodiment is similar in manyrespects to the embodiment of FIG. 33. Thus, for simplicity ofdescription, reference numerals are shared for the common elements andthe description below addresses only the differences therebetween. Inthe embodiment of FIG. 34, the bottom E1 electrode is realized by anexternal insulating layer 103-4 and electrode 1024 mounted on theexterior of the drill collar 16″. The electrode 102-4 may be realized ametal ring embedded in the insulating layer 103-4. Similar to theexternal electrodes mounted above the bottom electrode 102-4, theexternal insulating layer 103-4 and the electrode 102-4 can be mountedflush with the exterior of the drill collar 16″ or slightly recessed toprevent damage. Wearbands with a slightly larger diameter than the drillcollar 16″ can also be added to prevent damage to the electrodes. Theinsulation layer 103/4 insulates the E1 electrode 1024 from the body ofthe drill collar 16″. The electrical extender 110″, the electricalsocket 111″ and the bulkhead 116 connect the electrode 102-4 to themeasurement electronics that are housed in the annular chassis 107′.

Turning now to FIGS. 35A and 35B, an electrode of the tools describedherein, such as the E4 electrode 102-3′ of FIGS. 33 and 34, may besegmented into several separate electrodes, in order to measureazimuthal variations of streaming potential. FIGS. 35A and 35B show fourelectrodes (102-3A, 102-3B, 102-3C and 102-3D) distributed about thecircumference of drill collar 15F. The cross-sectional view of FIG. 35Ais perpendicular to the views in FIGS. 33 and 34. The view of FIG. 35Bshows only the insulation 103-3 and three of the four electrodes. Thefour electrodes are connected to the electronics by bulkheads and wires(not shown). Any of the four electrodes (E1, E2, E3 and E4) of the toolsdescribed herein may be so configured.

There have been described and illustrated herein several embodiments ofapparatus and methods for measuring streaming potentials andcharacterizing earth formation characteristics therefrom. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. Thus, while particular electrodearrangements have been disclosed, it will be appreciated thatmodifications can be made, provided the arrangement includes an array ofelectrodes capable of measuring streaming potentials. Thus, while it ispreferable to surface mount the electrodes and pressure sensor on thedrill string sections of the bottom hole assembly, it will be recognizedthat the electrode and pressure sensor may be mounted to the drillstring as part of a stabilizing collar or other collar assembly. It willbe appreciated that other configurations could be used as well. If thewell is drilled using casing drilling, liner drilling, or coiled tubingdrilling, it is understood that the same principles could be applied tothese other drilling methods. The electrodes, insulation, isolationjoint, and other aspects described herein could be implemented on casingsection, liner sections, or coiled tubing. It will therefore beappreciated by those skilled in the art that yet other modificationscould be made to the provided invention without deviating from itsspirit and scope as claimed.

1. A method for characterizing a pressure of an earth formationtraversed by a hole while drilling the hole comprising: i) varying thebottom hole pressure; ii) measuring a plurality of different bottom holepressures; iii) deriving a DC component of a streaming potential voltagesignal corresponding to each one of said plurality of different bottomhole pressures; iv) fitting the DC components and corresponding bottomhole pressures to a linear equation represented by a straight line; v)using said linear equation to derive an estimate of the pressure of theearth formation.
 2. A method according to claim 1, wherein: in i), thebottom hole pressure is varied by varying circulation rate of mud in thehole.
 3. A method according to claim 1, wherein: in i) the bottom holepressure is varied by varying the weight of the mud circulating in thehole.
 4. A method according to claim 1, wherein: in v), said linearequation is solved to identify a particular bottom hole pressure valuethat corresponds to a DC component value that is substantially zero, andsaid particular bottom hole pressure value is output as the estimate ofthe pressure of the earth formation.